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Remote LDP LSP Fails to Be Established Between Routers of Different IS-IS Levels

Publication Date:  2013-09-03 Views:  41 Downloads:  0
Issue Description

Scenario: On a large backbone network, an IGP area needs to be divided into multiple areas.
The networking is as follows:
R1 (IS-IS level-1)-----R2 (level-1-2)-----R3 (level-2)---R4 (level-1-2)-----R5 (level-1)
R1 and R2 are located in the same area, and R4 and R5 are located in the same area. R1 and R5 are level-1 routers, R2 and R4 are level-1-2 routers, and R3 is the level-2 router.


A remote LDP peer relationship is established between R1 and R5. The status of the LDP session is Operational.

ldp session(s) in public network
codes: lam(label advertisement mode), ssnage unit(dddd:hh:mm)
a '*' before a session means the session is being deleted.
------------------------------------------------------------------------------
peerid status lam ssnrole ssnage kasent/rcv
------------------------------------------------------------------------------
2.2.2.2:0 operational du active 0000:03:02 730/730
5.5.5.5:0 operational du passive 0000:03:02 729/729
------------------------------------------------------------------------------
total: 2 session(s) found.

The LDP LSP fails to be established.

ldp lsp information
-------------------------------------------------------------------------------
destaddress/mask in/outlabel upstreampeer nexthop outinterface
-------------------------------------------------------------------------------
*5.5.5.5/32 liberal
*5.5.5.5/32 liberal
Handling Process

1. Check the remote LDP peer configuration.
mpls ldp remote-peer r5
remote-ip 5.5.5.5
undo remote-ip pwe3

mpls ldp remote-peer r1
remote-ip 1.1.1.1
undo remote-ip pwe3

The remote LDP peer configuration is correct.

2. Check the IGP configuration and routing table. The IGP configuration is correct, and the IP routing table and IS-IS route are normal.
IP routing table:

route flags: r - relay, d - download to fib
------------------------------------------------------------------------------
routing tables: public
destinations : 9 routes : 9

destination/mask proto pre cost flags nexthop interface

0.0.0.0/0 isis 15 10 d 23.1.1.1 gigabitethernet0/0/0

IS-IS route:

route information for isis(1)
-----------------------------

isis(1) level-1 forwarding table
--------------------------------

ipv4 destination intcost extcost exitinterface nexthop flags
----------------------------------------------------------------------------
0.0.0.0/0 10 null ge0/0/0 12.1.1.1 a/-/-/-

3. In the IP routing table, no 32-bit host route is available to 5.5.5.5. The LDP triggering mode is host. Therefore, the remote LDP peer relationship can be established only when the 32-bit address of the remote peer exists in the routing table.
Root Cause

Routers in different IS-IS areas do not have 32-bit host routes to each other.

The LDP triggering mode is host. Therefore, the remote LDP peer relationship can be established only when the 32-bit address of the remote peer exists in the routing table.
Solution
1. On R2 and R4, run the import-route isis level-2 into level-1 command to perform route leaking so that R1 and R5 can learn the loopback address from each other.
2. On R1 and R5, configure static routes with loopback addresses as the destination addresses.
3. Level-1 routers learn too many routes to prevent route leaking, which increases processing loads. Configure the route leaking policy to prevent level-1 routers from leaking only the routes of 32-bit loopback addresses.
Suggestions
1. Check whether the MPLS, LDP, and IGP configurations are correct.
2. Check whether routes can be learned.
3. Check whether routes between R1 and R5 are reachable.
4. Check whether the remote LDP configuration is correct.

END