Mpls Ldp Session Protection

A common problem in networks is flapping links. The flapping of links can have several causes, but it is not the goal of this book to look deeper into this. Flapping links do have an important impact on the convergence of the network. Because the IGP adjacency and the LDP session are running across the link, they go down when the link goes down. This is unfortunate, especially because the link is usually not down for long. The impact is pretty severe though, because the routing protocol and LDP can take time to rebuild the neighborship. LDP has to rebuild the LDP session and must exchange the label bindings again. To avoid having to rebuild the LDP session altogether, you can protect it. When the LDP session between two directly connected LSRs is protected, a targeted LDP session is built between the two LSRs. When the directly connected link does go down between the two LSRs, the targeted LDP session is kept up as long as an alternative path exists between the two LSRs. The LDP link adjacency is removed when the link goes down, but the targeted adjacency keeps the LDP session up. When the link comes back up, the LSR does not need to re-establish the LDP session; therefore, the convergence is better. The global command to enable LDP Session Protection is this:

mpls ldp session protection [vrf vpn-name] [for acl] [duration seconds]

The access list (acl) you can configure lets you specify the LDP peers that should be protected. It should hold the LDP Router Identifier of the LDP neighbors that need protection. The duration is the time that the protection (the targeted LDP session) should remain in place after the LDP link adjacency has gone down. The default value is infinite.

For the protection to work, you need to enable it on both the LSRs. If this is not possible, you can enable it on one LSR, and the other LSR can accept the targeted LDP Hellos by configuring the command mpls ldp discovery targeted-hello accept.

Look at Figure 4-9 to see an example. LDP Session Protection is enabled on all four routers. The LSR madrid has two LDP sessions: one with london and one with sydney. When the link madridsydney fails, the targeted LDP session is held up as it reroutes over the path madrid-london-rome-sydney. Example 4-30 shows the LDP session on madrid to router sydney before the link went down. The link madrid-sydney then goes down. You can see a logging message for the LDP session when the link goes down and when the link comes back up. The first logging message indicates that the LDP session has gone into protecting state; the second indicates that the LDP session has been recovered successfully.

Figure 4-9 LDP Session Protection

Loopback 0 10.200.254.5/32

Example 4-30 Example of LDP Session Protection madrid#show mpls ldp neighbor serial 4/0 detail

Peer LDP Ident: 10.200.254.4:0; Local LDP Ident 10.200.254.5:0 TCP connection: 10.200.254.4.646 - 10.200.254.5.21396 State: Oper; Msgs sent/rcvd: 43/42; Downstream; Last TIB rev sent 63 Up time: 00:15:32; UID: 18; Peer Id 0; LDP discovery sources:

Targeted Hello 10.200.254.5 -> 10.200.254.4, active, passive;

holdtime: infinite, hello interval: 10000 ms Serial4/0; Src IP addr: 10.200.216.2

holdtime: 15000 ms, hello interval: 5000 ms continues

Example 4-30 Example of LDP Session Protection (Continued)

Addresses bound to peer LDP Ident:

10.200.254.4 10.200.214.2 10.200.217.1 10.200.216.2 Peer holdtime: 180000 ms; KA interval: 60000 ms; Peer state: estab Clients: Dir Adj Client

LDP Session Protection enabled, state: Ready duration: infinite madrid#show mpls ldp discovery

Local LDP Identifier: 10.200.254.5:0 Discovery Sources: Interfaces:

Ethernet3/1 (ldp): xmit/recv LDP Id: 10.200.254.2:0

Serial4/0 (ldp): xmit/recv

LDP Id: 10.200.254.4:0 Targeted Hellos:

10.200.254.5 -> 10.200.254.4 (ldp): active/passive, xmit/recv

LDP Id: 10.200.254.4:0 10.200.254.5 -> 10.200.254.2 (ldp): active/passive, xmit/recv LDP Id: 10.200.254.2:0

madrid#

02:48:38: %OSPF-5-ADJCHG: Process 1, Nbr 10.200.254.4 on Serial4/0 from FULL to DOWN, Neighbor Down: Interface down or detached

02:48:39: %LINK-3-UPDOWN: Interface Serial4/0, changed state to down 02:48:39: %LDP-5-SP: 10.200.254.4:0: session hold up initiated

02:48:40: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial4/0, changed state to down madrid#show mpls ldp neighbor 10.200.254.4 detail

Peer LDP Ident: 10.200.254.4:0; Local LDP Ident 10.200.254.5:0 TCP connection: 10.200.254.4.646 - 10.200.254.5.21396 State: Oper; Msgs sent/rcvd: 55/51; Downstream; Last TIB rev sent 69 Up time: 00:17:18; UID: 18; Peer Id 0; LDP discovery sources:

Targeted Hello 10.200.254.5 -> 10.200.254.4, active, passive; holdtime: infinite, hello interval: 10000 ms Addresses bound to peer LDP Ident:

10.200.254.4 10.200.214.2 10.200.217.1 10.200.216.2 Peer holdtime: 180000 ms; KA interval: 60000 ms; Peer state: estab Clients: Dir Adj Client

LDP Session Protection enabled, state: Protecting duration: infinite madrid#

02:49:10: %LINK-3-UPDOWN: Interface Serial4/0, changed state to up

02:49:11: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial4/0, changed state to up

02:49:15: %LDP-5-SP: 10.200.254.4:0: session recovery succeeded

Chapter Review Questions 103

Finally, a useful LDP feature is LDP Graceful Restart. It specifies a mechanism for LDP peers to preserve the MPLS forwarding state when the LDP session goes down. As such, traffic can continue to be forwarded without interruption, even when the LDP session restarts. You can find more information on LDP Graceful Restart in "Chapter 4 Supplement" at http:// www.ciscopress.com/title/1587051974.

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