I have 2 Cisco IOS connect to R1 and R3:
CE1 --- R1 ---tunnel--- R3 --- CE3
CE1:
interface Ethernet0/0
ip address 1.1.1.100 255.255.255.0
ip route 3.3.3.0 255.255.255.0 1.1.1.1
CE3
interface Ethernet0/0
ip address 3.3.3.100 255.255.255.0
ip route 1.1.1.0 255.255.255.0 3.3.3.3
CE01-GRE#ping 3.3.3.100
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.100, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 2/4/10 ms
CE01-GRE#
Traffic between R1-R2:
Traffic between R2-R3:
You can see that TTL in the inner IP header remains unchanged when the packet traversing through the GRE tunnel.
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Harry
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Original Message:
Sent: 09-24-2021 02:21
From: Unknown User
Subject: TTL in inner IP header GRE tunnel
Hi.
Do you mind running the test with transit traffic i.e. pinging from behind GRE ingress R1 ?
Thanks.
Original Message:
Sent: 09-23-2021 12:24
From: NAM NGUYEN
Subject: TTL in inner IP header GRE tunnel
I don't know if I am missing some things.
This tutorial is from Juniper Official Training.
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NAM NGUYEN
Original Message:
Sent: 09-23-2021 11:05
From: Unknown User
Subject: TTL in inner IP header GRE tunnel
As traffic goes through the transit devices, the TTL of the outer header is decremented as per regular IP forwarding rules, the inner IP header is not, if the book suggests otherwise, please check the errata of the book and see if there is any correction on it.
Elvin
Original Message:
Sent: 09-23-2021 05:24
From: NAM NGUYEN
Subject: TTL in inner IP header GRE tunnel
Hello all,
I read in JNCIS course:
"The inner packet (also known as the payload packet) is not
modified, except the time-to-live (TTL) field, which is decremented. The TTL field must be
decremented to ensure that the packet does not live forever."
But I build a VMX lab to test and see that TTL in the inner IP header doesn't change when packet traversing in the tunnel.
My topology:
R1# show interfaces gr-0/0/0
Sep 20 23:54:05
description TEST_GRE_TUNNEL;
unit 0 {
tunnel {
source 10.249.50.84;
destination 10.249.50.86;
}
family inet {
address 192.168.12.1/24;
}
}
[edit]
R1# show routing-options
Sep 23 01:44:20
static {
route 3.3.3.0/24 next-hop gr-0/0/0.0;
}
===
R3# show interfaces gr-0/0/0
Sep 20 23:54:05
description TEST_GRE_TUNNEL;
unit 0 {
tunnel {
source 10.249.50.86;
destination 10.249.50.84;
}
family inet {
address 192.168.12.3/24;
}
}
R3# show routing-options
Sep 23 01:44:20
static {
route 1.1.1.1.0/24 next-hop gr-0/0/0.0;
}
R1#ping 3.3.3.3 source 1.1.1.1
Sep 20 23:58:04
PING 3.3.3.3 (3.3.3.3): 56 data bytes
64 bytes from 3.3.3.3: icmp_seq=0 ttl=64 time=2.911 ms
64 bytes from 3.3.3.3: icmp_seq=1 ttl=64 time=95.914 ms
Capture packet from R1 to R2:
Capture packet from R2 to R3:
As you see, TTL in the inner IP header remains unchanged (still be 64) when packet traversing in the tunnel.
Only TTL in the outer is decreased.
Hope you clear this up for me.
Thank you.
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NAM NGUYEN
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