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Afroz Ahmad

My Official Blog


Category: MPLS

MPLS Terminologies

>MPLS:-The evolution of MPLS was Cisco’s TAG Switching however as it seems
it was Cisco’s proprietary so it must be used in Cisco world only,to make it
open standard IEEE made a similar kind of Protocol not as similar as Cisco’s
Tag switching however the idea was the same and they named it MPLS.MPLS is a
forwarding mechanism in which packets are forwarded based on labels,it means
no more routing lookups only labels are looked and forwarded in the ISP cloud,
so the movement becomes faster and the security enhanced.
As it was the new idea so the new terms are also attached with the logic behind
the MPLS.
>LER :-Label Edge Router,is the router who is imposing or disposing the labels on
the each individual routes coming in or going out of the ISP cloud.LER is
commonly known as PE or Provider Edge Router.
>Ingress LER :-It is the PE router who is imposing the label in front of the IP
>Egress LER :-It is the PE router who is disposing the label and forwarding the IP
packet without label.
>LSR :-Label Switch Router,is a router that is doing forwarding of the packets
based on labels,also swapping the labels and tagging them with a new label,
commonly known as P or Provider router.
>CE :-Customer Edge router , is client site router connected to the ISP’s
MPLS cloud.
>Label :-It is 4-byte(32 bit,same bit as in IP packet) identifier used in MPLS to
make forwarding decisions.
>Label Binding :-Mapping a label to a FEC.
>FEC :-It is a group of packets forwarded in the same manner,over the same path,
or with same forwarding treatment.
>LSP :-Label Switch Path , is series of LSR’s that forward labeled packets based
on the FEC.
>PHP :- Penultimate-Hop-Popping,it is a mechanism where the popping(removing) of
label is done one hop before the Egress PE router.
>RIB :- (Routing Information Base)It is like the Datacentre of all the routes
available in the router,that is further transferred into FIB for further forwarding.
>LIB :- (Label Information Base)It is like the Datacentre of all the labels bound to
networks available in the router,the labels are further move from LIB to LFIB for
further forwarding.
You can check the LIB with the “Show mpls ldp bindings”
>FIB :- (Forwarding Information Base)Is a CEF(Cisco Express Forwarding) build table
from the information in the RIB,used for forwarding.
You can check the FIB with the “Show ip route”
>LFIB :- (Label Forwarding Information Base)Is a CEF table used for forward labeled
packets,the LFIB only stores the labels used to forward packets.
You can check LFIB with the “sh ip mpls forwarding-table”
>Control Plane :-Uses the configured routing protocols to build a routing table,
called the RIB,uses a label exchange protocol to maintain labels internally in a
table called the LIB.Is also responsible for building two tables in the Forwarding
Plane,the FIB and the LFIB tables.
>Data/Forwarding Plane :-It consists of two tables,the FIB and LFIB which is
responsible for forwarding incoming packets either based on IP(unlabeled)or
using the label.It is also responsible to push/pop/swap of labels.

>LDP:- Label Distribution Protocol used for transporting labels into MPLS cloud,IEEE standard.
>TDP:- Tag Distribution Protocol it is also used for transporting labels into MPLS
 cloud however it is Cisco’s proprietary.

Will be discussing the above topics scenario-wise in coming posts, so do watchout 🙂

OSPF /32 issue in MPLS VPN Scenario

Yesterday i and my office colleague were doing a MPLS-VPN scenario,where we stuck
at a place , the issue was really good , thats why thought of posting it on my blog.

The scenario was like


Here in this scenario Eigrp is running between R1 and R3 and R5 and R2.we have
taken vrf name RED,also OSPF is running as underline IGP between R3–R4–R5,here
BGP is not running on R4.
The loopback interfaces addresses are taken from the device id like for R1 the
loopback is likewise till R2
We have configured everything properly as per the diagram,means we have correctly
defined “ip vrf RED” with “RD 100:1” and imported and exported correctly from and to
BGP with “route-target export 100:1” and “route-target import 100:1” at both PE ends
Correctly defined the vpnv4 and ipv4 address-families in BGP,means everything was
configured correctly and with high accuracy.
Now here comes the issue we were seeing loopback routes in R1 from R2 and vice-versa.
It means that we have configured correctly however when we tried to ping ,the ping
was failing,we have rechecked the configuration by:-
R3#sh ip bgp vpnv4 all
All the routes were marked with *> means they are best routes indeed.
>We checked at both PE’s R3 and R5.
R3#sh ip route vrf RED
The routes were received properly.
>Also checked the MPLS LIB and LFIB.
R3#sh ip mpls ldp bindings
R5#sh ip mpls forwarding-table
They were also seems to look good.
>Even we checked the CEF table.
R3#sh ip cef
R3#sh ip cef vrf [NAME] {prefix} detail
>However the issue was same we were still not able to ping end to end.

After around two hours we have atlast figured the issue.
Here are the details.
>I was running OSPF as IGP between P/PE routers in the MPLS backbone and using
loopback interfaces on the PE routers for BGP updates and for testing.I did make these
interfaces use a /24 mask.The configuration is looks like
R3#show run
interface loopback 0
ip address
router ospf 1
network area 0

Checking the routing table on the second PE(R5) would show that a /32 mask has been
advertised for the network rather than as i had intended.

R5#show ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS
ia – IS-IS inter area, * – candidate default, U – per-user static
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set is subnetted, 1 subnets
O [110/3] via, 00:03:55, FastEthernet 1/0

It is therefor not as i intended in the MPLS forwarding table as a /24 either.

R5#show mpls forwarding-table
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched    interface
16     16 0          Fa1/0   
17     Untagged 0          Fa1/0
18     Pop tag 0      Fa1/0  

Although then i recalled my OSPF studies that this can be resolved by issuing
“ip ospf network point-to-point”command under the loopback interfaces in P/PE’s.
Or by simply taking /32 mask for loopback interfaces,because we are using OSPF and
in OSPF either we should use..”network point-to-point” under loopback interface or
use /32 mask ,because even if we are taking /24 mask the ospf will advertise
/32 mask by there may be instability in routing table..

Always watch for simple things first before going into complex scenarios 🙂