MPLS

  1. MPLS
    1. MPLS LSP
    2. MPLS EXP & TTL
    3. MPLS shim
  2. LDP
    1. Session negotiation
    2. LDP PDU
      1. LDP TLV
      2. LDP message
      3. FEC TLV
      4. Generic label TLV
      5. Address list TLV
      6. Hop count TLV
      7. Path vector TLV
      8. Status TLV
      9. Common hello parameters TLV
      10. Common session parameters TLV
    3. LDP TLVs
    4. CLI
    5. LDP session protection
    6. LDP autoconfiguration
    7. LDP IGP sync
  3. Multicast LDP
  4. Labels
  5. IP aggregarion
  6. Graceful restart
  7. TTL expiration
  8. MPLS payload
  9. Maximum receive unit (MRU)
  10. Filtering
  11. QoS
  12. MPLS CEF LB
  13. 6PE
  14. Carrier’s Carrier (CsC)
  15. Inter-AS VPN Option B
  16. Inter-AS VPN Option C (multihop MP-BGP)
  17. 2547oDMVPN
  18. Load-balancing

MPLS

  • switching packets based on label value
  • Ethertype = 0x8847 (unicast)
  • based on unicast RIB, label per prefix (for BGP – label per next-hop ⇒ next-hop-self required)
  • IGP + LDP
  • TTL:
    1. IP header TTL does not change across LSP
    2. ingress E-LSR copies IP TTL to MPLS TTL, every router decreases MPLS TTL, egress E-LSR copies MPLS TTL to IP TTL
    3. decreased on label swap
    4. not copied if MPLS TTL > IP TTL (loop safeguard)
    5. TTL proparation has to be disabled either on all or none routers
  • PHP:
    • penultimate hop popping = before ultimate hop
    • removes label before sending to E-LSR
    • 1 lookup instead of 2 (no label lookup in LFIB)
    • implicit null ≡ pop label
    • only for connected and aggregated (they 100% require IP lookup for forwarding)
  • load-balancing only across MPLS links, unlabeled IP path not used (otherwise drop, e.g. AToM)
  • LIB stores all labels, LFIB – only active
  • FEC – forwarding equivalence class (~ IP dst, mcast group, …) – packets for the same class are sent across the same LSP
  • mode:
    1. frame mode
    2. cell mode (ATM)
  • label mode:
    1. liberal:
      • stores labels from active neighbours even if not used (not next-hop)
      • speeds up convergence
      • not LC-ATM
    2. conservative:
      • stores only active labels
      • less RAM utilization
      • LC-ATM
  • LSP control mode:
    1. independent: labels are generated after receiving FEC
    2. ordered: labels are generated after receiving FEC and next-hop label
    3. ATM switches only
  • IP precedence is copied to EXP on PE; optionally EXP can be copied to IP precedence
  • no outgoing label = pop labels + FIB IP
  • label unknown → drop

MPLS LSP

Labels are swapped when crossing LSR

PE1 → PE2 – label switched path (LSP)

LSP = 22;39

LSP – unidirectional

Using PHP removes LFIB lookup on PE2

MPLS EXP & TTL

MPLS shim

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Label                   | EXP |S|      TTL      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Label: 0-15 reserved
EXP: undefined (deprecated RFC), QoS (active RFC)
S = 1: bottom of stack, for first label after IP header
LSR – label switching router (push/pop label, switch)
E-LSR – edge LSR: ingress E-LSR (push), egress E-LSR (pop)

LDP

  • label distribution protocol
  • default (not TDP – deprecated)
  • MPLS TE – RSVP, MPLS VPN – MP-BGP, mcast – PIM or mLDP
  • announces labels for each prefix in uRIB; exception – BGP: per next-hop generation, generated by next-hop only on putting itself as NEXT_HOP
  • trigger for allocation: new route in RIB
  • LIB: MPLS label information base
  • messages:
    1. Hello:
      • 224.0.0.2, UDP 646
      • LDP ID, label space (= 0), holdtime
      • announces IP address for TCP connection (ID or explicit transport address)
    2. Update:
      • unicast, TCP
      • announces mapping label-FEC
    3. Initialization:
      • TCP
      • label distribution method, keepalive time, TLV, timers for TCP
    4. Notification:
      • TCP
      • error (e.g. parameters not compatible)
    5. Keepalive:
      • TCP
      • refreshes TCP session
      • acknowledge receiving Initialization
    6. Address / Withdraw address:
      • TCP
      • announces bound address
    7. Label / Label withdraw:
      • TCP
      • announces labels
  • PHP (pop label): connected and aggregated only; for other prefixes – egress LSR generates labels
  • label distribution:
    1. downstream on demand (DoD):
      • label only from next-hop
      • LC-ATM
    2. unsolicited downstream:
      • label from all adjacent neighbours
      • not LC-ATM
  • does not allocate labels for BGP prefixes (IOS)
  • TCP session is started by highest LDP IP:
    1. manual configuration
    2. highest loopback
    3. highest physical
  • if there are several links between LSR within same label space, they have to use the same transport address; otherwise only 1 TCP to the neighbour, no load-balancing (1 interface in discovery sources = only 1 link for LDP neighbouship)
  • IOS: label allocation is asynchronous, does not depend on others
  • targeted Hello: sent unicast if neighbour is not directly connected (TE tunnel)
  • timers:
    1. hello: 5s, 3 times lower than holdtime in use
    2. keepalive: 60s
    3. holdtime:
      • 180s for TCP
      • 15s for UDP
      • if neighbours have different values, lower value is used

Session negotiation

Label space:

  • 2 LSB bytes of LDP ID
  • 0 ≡ per-platform, otherwise space number

LDP PDU

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Version           |           PDU length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            LSR ID                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Label space ID        |                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         TLV/message           /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LDP TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F|           Type            |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                             Value                             /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

U: unknown bit; 0 ≡ on recognising as unknown – send notification; 1 ≡ ignore if unknown
F: forward unknown bit; 1 ≡ forward if the message is to be forwarded

LDP message

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|        Message type         |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Message ID                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                      Mandatory parameters                     /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                       Optional parameters                     /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Types:

  • Notification: 0x0001
  • Hello: 0x0100
  • Init: 0x0200
  • Keepalive: 0x0201
  • Address: 0x0300
  • Address Withdraw: 0x0301
  • Label Mapping: 0x0400
  • Label Request: 0x0401
  • Label Abort: 0x0404
  • Label Withdraw: 0x0402
  • Label release: 0x0403
  • Vendor private: 0x3E00 – 0x3EFF
  • MAC flush: 0x0406

FEC TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|          0x0100           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    FEC type   |              AFI              | Prefix length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\                                                               \
/                             Prefix                            /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

FEC type:

  • 1: wildcard (no payload)
  • 2: prefix
  • mcast:
    • 6: P2MP
    • 7: MP2MP-up
    • 8: MP2MP-down
    • prefix = root address
    • at the end of FEC element – opaque length + value

Generic label TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|          0x0200           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Label                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Address list TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|          0x1001           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              AFI              |                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           Addresses           /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Hop count TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|          0x1003           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Hop count   |
+-+-+-+-+-+-+-+-+

Path vector TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|          0x0104           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            LSR ID 1                           | 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              ...                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            LSR ID n                           | 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Status TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F|          0x0300           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|F|                      Status data                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Message ID                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Message type           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

U: 0 ≡ in Notification, 1 ≡ in other messages
E: 1 ≡ fatal error, 0 ≡ advisory notification
F: forward bit, 1 ≡ forward message
Message type / ID determine message-trigger (0 ≡ generated as is, not as a response for message)

Common hello parameters TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|          0x0400           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Hold time (seconds)     |T|R|         Reserved          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

T: 1 ≡ targeted hello
R: 1 ≡ request targeted Hello

Common session parameters TLV

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|          0x0500           |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Version            |   Keepalive timer (seconds)   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|D| Reserved  |     PVLim     |         Max PDU length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            LSR ID                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Label space ID        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A: 0 ≡ DU, 1 ≡ DoD
D: loop detection (0 ≡ disabled)
PVLim: path vector limit (0 if D=0)

LDP TLVs

  • 0x0401: IPv4 transport address
  • 0x0402: configuration sequence number (4 bytes)
  • 0x0403: IPv6 transport address
  • 0x0202: FR label
  • 0x0201: ATM label
  • 0x0501: ATM session parameters
  • 0x0502: FR session parameters

CLI

; ingress E-LSR does not copy from IP TTL, using 255 instead
; egress E-LSR does not copy into IP TTL
; masquerade form traceroute
(config)# no mpls ip propagate-ttl [forwarded|local]

(config)# ip cef
(config)# mpls ip

; by default MPLS does not allcoate label for 0.0.0.0/0; enables for IGP routes
(config)# mpls ip default-route

; force makes router change RID immediately
(config)# mpls ldp router-id <intf> [force]

; TCP
(config)# mpls ldp holdtime <sec>

; ACL – prefixes, PEER – ACL for neighbours based on RID, interface – as a local route, /32 prefix from interface prefix
(config)# mpls ldp advertise-labels [interface <intf>] [for <ACL>] [to <PEER>]

; UDP
(config)# mpls ldp discovery hello holdtime|interval <sec>

; instead of IP in LDP ID
(config)# mpls ldp discovery transport-address <intf>|<IP>

; init, max – in seconds, exponentially increases timeout for TCP session reestablishment within [init; max]
(config)# mpls ldp backoff <init> <max>

(config)# mpls label protocol ldp

; label pool, 16-1000000 by default
(config)# mpls label range <min> <max>

; disables sending Label Withdraw before Label with new tag
(config)# mpls ldp neighbor <IP> implicit-withdraw

(config)# mpls ldp neighbor <IP> password <PASSWD>

; explicit-null in lieu of implicit-null
(config)# mpls ldp explicit-null [for <PREFIX>] [to <PEER>]
(config-if)# mpls ip

; size of packet with labels (IP MTU + n*4)
(config-if)#mpls mtu <MTU>
# show mpls forwarding-table [<prefix> <length>]

# show mpls interfaces

# show mpls ldp bindings <prefix> <length>

# show mpls ldp neighbor

; OOR: out-of-resource
# show mpls ldp parameters

# show mpls ldp interfaces

# show mpls label range
# debug mpls packet

# debug mpls ldp advertisements

LDP session protection

  • stores info about neighbour labels if link goes down; MPLS can converge as soon as IP converges
  • 24h default
  • on link down targeted Hello are sent; at least 9 Hello are sent before tearing down TCP and neighbourship
  • difference from targeted Hello: unicast neighbourship is limited in time
  • useful for link flaps
; ACL – standard, prefixes that are protected
(config)# mpls ldp session protection [for <ACL>] [duration <sec>]

; enabling targeted Hello (without protection, e.g. for TE), both sides, ~ static neighbour
(config)# mpls ldp neighbor <IP> targeted ldp

; enables answering targeted Hello from several neighbours
(config)# mpls ldp discovery targeted-hello accept [from <ACL>]

(config)# mpls ldp discovery targeted-hello holdtime <sec>

(config)# mpls ldp discovery targeted-hello interval <sec>

LDP autoconfiguration

  • autoenabling LDP, MPLS on IGP-enabled interface
  • supports OSPF, IS-IS
  • interface belongs to IGP
  • must be enabled for single IGP only
(config-router)# mpls ldp autoconfig [area <n>]
(config-if)# no mpls ldp igp autoconfig

LDP IGP sync

  • IGP announces proper info about the link only after LDP converges (sends worst cost for the link before that)
  • MPLS traffic can be dropped if sync not enabled (IGP built the path but labels are not negotiated)
  • convergence:
    1. at least 1 binding sent
    2. at least 1 binding received
  • path through unsynchronized link is used only when it is the only path
  • supports OSPF (cost = 0xFFFF), IS-IS (cost = 0xFFFFFE)
  • if OSPF did not form adjacency unless LDP was up, there would be a deadlock (tranport address not reachable)
; enables sync on all IGP interfaces
(config-router)# mpls ldp sync
(config-if)# no mpls ldp igp sync
; how long to wait for LDP sync; infinity by default, when expires – announce proper cost
(config)# mpls ldp igp sync holddown <ms>
# show mpls ldp igp sync

# show ip ospf mpls ldp interface <intf>
# debug mpls ldp sync [interface <intf>] [peer-acl <ACL>]

Multicast LDP

  • LDP extension
  • LSP:
    1. P2MP:
      • TLV 0x0508
      • data MDT, PE – root, only PE sends downstream
    2. MP2MP:
      • TLV 0x0509
      • default MDT, MDT number = 0, PEs send and receive from RPT
  • MP-LSP are created once receiver is available (pull model)
  • DoD label allocation, conservative mode, no PHP (~ VPN label)
  • MP-LSP ­– tree with root in MPLS core
  • FEC:
    1. type: P2P, BD
    2. AF
    3. address length
    4. root address
    5. opaque:
      • IPv4/v6 SSM transit
      • mcast VPN
      • direct MDT – VPN without default MDT
  • PIM overlay: PE see each other as PIM neighbors through LSP-VIF
(config)# ip pim mpls source <lo>

; can use mLDP on top of TE
(config)# mpls mldp path traffic-eng

; use route through non-TE for RPF (mcast is forwarded natively)
(config)# mpls traffic-eng multicast-intact
; ID ≈ group address default MDT in draft Rosen (GRE); RD can be used
(config-vrf)# vpn id <ID>
; set IP MP-LSP root
(config-vrf-af)# mdt default mpls mldp <root>

; num = labels in pool for data MDT
(config-vrf-af)# mdt data mpls mldp <num>

; default = infinity
(config-vrf-af)# mdt threshold <kbps>
# show mpls mldp neighbors

; default MDT root
# show mpls mldp root

; MP2MP labels
# show mpls mldp bindings

; data MDT labels
# show mpls mldp database [summary]

Labels

  • 0:
    • explicit-null IPv4
    • for preserving DSCP, instead of implicit-null
    • just poped, not involved in forwarding (can be announced for different FEC)
  • 1:
    • router alert
    • always top label
    • makes router process payload
  • 2:
    • explicit-null IPv6
  • 3:
    • implicit-null
    • not used as an actual label (PHP only)
  • 7:
    • entropy label indication
    • TTL = 0
    • used for load-balancing, next label – entropy label
  • 13:
    • GAL: generic associated channel label
    • before GACH, bottom of stack
  • 14:
    • OAM alert
    • RFC 3429
    • distinguishes OAM packets from regular ones
    • not used by IOS

IP aggregarion

LSP is split into 2 pieces at aggregation point

B does not know about /24 so it does not use corresponding label. As a result, B and C perform extra lookup

Undesirable for end-to-end LSP: VPN, AToM, TE

Graceful restart

  • control plane recover without disrupting data plane
  • timers:
    1. reconnect;
      • 120s default
      • how long to wait for neighbour to reestablish the session, otherwise – reset
    2. holdtime:
      • 120s default
      • stores info from neighbour till rewrite/reset
; enable before establishing LDP session
(config)# mpls ldp graceful-restart

; storing info about dataplane after control plane failure
(config)# mpls ldp graceful-restart timers forwarding-holding <sec>

; holdtime, after reconnect success
(config)# mpls ldp graceful-restart timers max-recovery <sec>

; reconnect
(config)# mpls ldp graceful-restart timers neighbor-liveness <sec>

TTL expiration

  • ICMP responses are sent further along LSP so that PE/CE sends them back (P does not know address)
  • TTL propagation has to be disabled along the whole LSP (otherwise drop on P)
  • ICMP time exceeded is generated only for IPv4/IPv6; for AToM – drop in general

MPLS payload

  • intermediate LSR does not use payload, just swaps labels
  • egress E-LSR knows about payload when it allocates label ⇒ can determine payload type by labe;
  • egress E-LSR copies IP precedence into EXP of all labels by default

Maximum receive unit (MRU)

  • per FEC
  • 2 labels along LSP:
    1. for P-router MRU = 1508 = MTU
    2. for egress E-LSR MRU = 1512 (MTU = 1508, PHP)
    3. for ingress E-LSR MRU = 1504 (label would be pushed later)
(config)# system mtu <MTU>

(config)# system jumbomtu <MTU>

Filtering

; disables advertising labels to everyone (by default)
(config)# no mpls ldp advertise-labels

; allocate label for IP/32 form the interface if such a prefix not in RIB
(config)# mpls ldp advertise-labels interface <intf>

; ACLs – standard, PREFIX does not use prefix mask (1.0.0.0/8 includes 1.0.0.0/25 as well, mask just for binary match), PEER – LSR ID
(config)# mpls ldp advertise-labels for <PREFIX> to <PEER>

; standard ACL, for prefixes, does not check for mask
(config)# mpls ldp neighbor <IP> label accept <ACL>

(config)# mpls ldp label
(config-ldp-lbl)# allocate global host-routes
(config-ldp-lbl)# allocate global prefix-list <PLIST>

QoS

  • modes:
    1. uniform: DSCP → EXP; PHB based on C-labels in P-core (managed CE scenario)
    2. short-pipe: DSCP ≠> EXP; PHB based on SP labels, on PE – on DSCP
    3. pipe: DSCP ≠> EXP; PHB on SP labels only (PE – on EXP in VPN label)
  • by default EXP is set from IPP/EXP on imposition/swap (disposition does not change lower EXP/IPP)
  • long pipe ≡ pipe in CsC

MPLS CEF LB

  1. payload = IPv4/v6: hash(src,dst)
  2. payload ≠ IPv4/v6: bottom label value
# show mpls forwarding-table labels <num> exact-path ipv4 <src> <dst>

6PE

  • labels for CE prefixes: MP-BGP, 1 label per IPv6 prefix
  • BGP next-hop: IPv4-mapped IPv6; enough because PE forwards packet based on label through MPLS IPv4 intf
  • two labels in stack: IPv6 + BGP next-hop (IPv6 is required because of PHP)
; allocates labels for IPv6 prefixes, same for IPv4, disabled by default for IPv4/v6, enabled for VPNv4/v6 by default, has to match on both neighbors
(config-router-af)# neighbor <IP> send-label
; IPv6 src for locally generated packets
(config)# mpls ipv6 source-interface <intf>

Carrier’s Carrier (CsC)

  • P routers do not know clients’ prefixes, only PE addresses
  • BGP session between CE
  • LDP between CE-PE
  • CE is placed into VRF on PE
  • inter-AS:
    1. back-to-back: ASBR = CE in VRF for another ASBR
    2. eBGP: ASBR generates its own VPN label for eBGP neighbor (ASBR – end of LSP, next-hop)
    3. multihop MP-eBGP with VPN:
      • ASBR are not connected directly
      • ASBR do not know VPNv4 prefixes – only next-hops
    4. multihop MP-eBGP between RR in each AS:
      • traffic does not pass through RR
      • ASBR do not know VPNv4 prefixes, only next-hops
    • link between ASBR – LDP or MPLS BGP forwarding (otherwise packets with VPN label are dropped)
    • BGP allocates and advertises own label only if it is next-hop ⇒ iBGP does not allocate label by default

R3, R5 known only the addresses of R1, R7

R1, R7 exchange prefixes via BGP

Inter-AS VPN Option B

If ASBR1 uses “redistribute connected” to announce BGP next-hop, PE1 cannot load-balance (will use MPLS through ASBR2 or ASBR3) since RD would be the same.

If ASBR1 uses “next-hop-self”, ASBR1 would be able to load-balance.

AddPath can also be used (IOS XR)

Inter-AS VPN Option C (multihop MP-BGP)

If ASBR sends update via eBGP, it inserts own label because it’s next-hop for the prefix. iBGP does not cause insertion ⇒ next-hop-self required

Label insertion trigger – inserting self as next-hop

Label allocation trigger through BGP when prefix not present in IGP RIB – route-map

; disable RT filtering on ASBR (drop unused RT by default)
(config-router)# no bgp default route-target filter

; on RR for RR in another AS, removes RR from LSP
(config-router)# neighbor <IP> next-hop-unchanged

; for iBGP to insert own label into Update (via becoming next-hop)
(config-router)# neighbor <IP> next-hop-self [all]
; allocates label for BGP prefix (!) and sends it in Update if next-hop
(config-route-map)# set mpls-label

; checks whether prefix has label assigned
(config-route-map)# match mpls-label
; enables MPLS, labels received via BGP
(config-if)# mpls bgp forwarding

2547oDMVPN

  • uses BGP to exchange labels instead of LDP
  • DMVPN phase 2
  • spoke-to-spoke GRE + MPLS inside
  • mcast – through hub only

If eBGP is configured between physical interfaces + “send-label”, then

  1. interfaces are configured automatically with “mpls bgp forwarding”
  2. connected /32 for IP address of neighbor is added to unicast RIB
  3. MPLS label is allocated

Load-balancing

  1. find BoS label
  2. if nibble = 4|6 – load-balance based on inner packet
  3. otherwise load-balance based on BoS label (VC label)